During locomotion, coordinated body, heat and ocular movements stabilize gaze and help maintain posture. An understanding of these movements requires 1) an analytical approach that relates the coordinate frames of the body, head and eyes during locomotion and 2) the development of a model that illustrates how these frames might be coordinated in real time. The goals of this study are to develop a general model-based approach for studying the kinematic and dynamic behavior of the body, arms, head and eyes during natural locomotion, relate this behavior to the workings of the angular and linear VCR's and VOR's, and determine bow stable locomotion is achieved through modeling the behavior. This will be accomplished by developing new computational methods for determining head fixation point (HFP) and gaze fixation point (GFP) in three dimensions while walking. It has been hypothesized that compensatory mechanisms maintain these points approximately invariant relative to the body. These tools will be used to study the compensatory responses of the head via the linear and angular vestibulocollic and vestibulo-ocular reflexes (1VCR, aVCR, IVCR, and IVOR). We will generalize our notion of the trajectory coordinate frame, utilized in the study of overground locomotion. to consider locomotion in 3-D, along grades and ascending and descending stairs. We will use our newly developed methodology to evaluate the quality of the compensation during circular locomotion, ascending and descending grades and stairs. It has also been postulated that orientation mechanisms tend to maintain alignment of head and ocular coordinate frames with the gravito-inertial acceleration (GIA). Thus, orientation effects due to changes in the GIA during these more general trajectories will also be studied. Arm swing will also be considered in these studies as a possible role for the cervico-collic reflex in regularizing locomotion. Experiments will be done to limit arm swing and determine its affect on body, head and eye movements. The effect of looking at targets while walking around a circular trajectory will also be studied. Finally, the data will be utilized to model the role of compensatory and orienting vestibular mechanisms during locomotion. The experiments proposed are experienced during natural locomotion in everyday life. While many aspects of body, arm, head and eye movements will be examined, they all will be monitored in focused experiments. The novelty is the representational schemes that will be developed and the model-based analysis that will relate this behavior to vestibular mechanisms. When this study is completed, we should have a better 3-D characterization of body, head and eye movements during a wide range of natural walking behaviors and have a clearer understanding of how the vestibular system generates compensatory gaze during locomotion and orients the body, head and eyes to maintain gait stablity.